The present invention relates to an electron beam apparatus allowing for an evaluation of a substrate as large as 8 inches or 12 inches with high precision (50 nm pixel to 25 nm pixel) as well as high throughput. Using a multi-beam is advantageous to achieve the high throughput, and an electron gun with a greater intensity of angular current is required to obtain the highly intensified multi-beam from a single electron gun, which may be represented by the electron gun of LaB6 and W hairpin having well-known angular current intensity values of 105 μA/Sr and 3.93×106 μA/Sr, respectively.
It can be seen from the comparison in the values of specific angular current intensity that the value of 104 μA/Sr for TaC is higher than 600 μA/Sr for ZrO/W but lower than 3.93×106 μA/sr for the W hairpin. The electron gun of TaC, however, emits intensive beams in the four different off-optical axial directions and this makes it easier to form the multi-beam. In this environment, the present invention is directed to an apparatus aimed at producing a multi-beam with a thermal electric field emission electron gun using a cathode made of TaC, for example, and also to a device manufacturing method which allows for the device to be manufactured with high yield by evaluating a wafer using the same electron beam apparatus.
In the background of this field of technology, for example, in regard to the emission of electrons from a carbide emitter of transition metal, Kumashiro, et al. have made a detailed report in their research paper, entitled “Electron emission of carbide emitter and surface thereof”, on the electron emission characteristics in an emitter employing TaC single crystal and a polycrystalline thermal emitter (see, the journal, “Applied physics”, Vol. 45, No. 7 (1976)).
As for the angular current intensity of the electron beam emitted from the thermal electric field emission electron gun, there are known values including, for example, 104 μA/Sr for the TaC and 600 μA/Sr for the ZrO/W. The W hairpin has a problem that a beam current could not be so high.
There is a problem in an electron gun of the Schottky cathode made of ZrO/W, LaB6 or carbide of transition metal that only a single beam along an optical axis has been used and inevitably a scanning operation with such a single beam consumes a lot of time.
Thus the present invention is directed to solve the above-described problem, or the problem that the long time is required to evaluate a large-sized substrate, such as an eight-inch wafer or a 12-inch wafer, with high precision (50 nm pixel to 25 nm pixel), and accordingly a first object of the present invention is to provide an electron beam apparatus allowing a number of electron beams, for example, four beams of electrons, to be produced for each optical axis yet with a relatively high current for every single beam of electrons.
The present invention further relates to an electron beam apparatus allowing for an image of a sample having a fine pattern to be taken with high resolution at a high rate.
In a practice to take a surface image of a sample, such as a wafer, according to the prior art, the sample surface is scanned with a narrowly converged electron beam and thus generated secondary electrons are detected to produce an SEM image for the purpose of enhancing the resolution of the image.
However, as the beam size is reduced, the beam current is also reduced in proportion to the fourth power of the dimension of beam diameter. To show one example, assuming that the pixel size is equal to the beam size, the following relations may be obtained:
Beam sizeBeam currentPixel frequencyFrame time (sec)0.1μmφ100nA100MHz1sec/mm20.05μmφ6.25nA6.25MHz64sec/mm225nmφ390PA390KHz4,096sec/mm210nmφ10PA10KHz1 × 106sec/mm2
Accordingly, since the beam current is reduced exponentially in association with the reduction of the beam size, the beam current of the secondary electrons emanating from a sample, such as a wafer, is reduced, which makes it impossible to increase an S/N ratio of the image. Due to this, there has been no other choice than decreasing the scanning rate over the sample with the beam of primary electrons, or decreasing the pixel frequency exponentially, in order to increase the emission of the secondary electrons, and this has led to a problem that a considerably long time period, that is exponentially increased, is required to scan the image of a finite area.
Thus the present invention is also directed to solve the above-indicated problem, or the problem that if the beam size is reduced in order to take the surface image of the sample, such as the wafer, with high resolution, then more time is required to take the image, and accordingly a second object of the present invention is to provide an electron beam apparatus that allows for a beam of electrons having a certain beam size larger than the conventional beam size to be used for scanning the sample, that takes an image at a high rate based on the secondary electrons emanating from the sample and resultantly produces the image with high resolution.
The present invention further relates to an electron beam apparatus that carries out a defect inspection and a defect review of a sample having a fine pattern defined by a minimum line width not greater than 0.1 μm with high throughput and also to a device manufacturing method using the same electron beam apparatus.
This electron beam apparatus is applicable to improving a process condition and to classifying a defective wafer by specifying a factor in developing a defect of bad conductivity based on an inspection result of the sample from a defect detecting apparatus. Thus, it is required to acquire the state of the defect of bad conductivity precisely in order to specify the factor in developing the defect of bad conductivity, and to address this, confirming a high resolution image identifying a location of the defect of bad conductivity, or a bad conductivity defect review, may be useful.
Conventionally, a defect detecting apparatus needs a relatively high beam current due to its nature that a high throughput is required, and thus has employed an electron optical system with a compromise resolution performance. On the other hand, a defect reviewing apparatus has employed an electron optical system with a compromising level of beam current due to its nature that a high resolution is required. Thus, since those two apparatuses use respective beams that are different from each other, there has been almost no apparatus operable satisfactorily to provide the defect evaluation in both of the defect detection and the defect reviewing.
If both of the defect detecting apparatus and the defect reviewing apparatus are installed in a clean room, another problem would be encountered that a large floor area should be necessary. There would occur an additional problem of large time-loss because after the detection of the defect by the defect detecting apparatus, the detected defect has to be searched for in the separate defect reviewing apparatus.
Accordingly, a third object of the present invention is to provide an integrated apparatus that can provide the defect detection and the defect reviewing in a serial manner in a small foot print. Another object of the present invention is to provide a device manufacturing method using the same apparatus.
The present invention further relates to an electron beam apparatus, in which a primary electron beam is irradiated on a sample, and secondary electrons or the like emitting from the sample are formed into an image in a image-projection optical system to thereby form a sample image.
In a conventional electron beam apparatus employing the image projection optical system according to the prior art, the primary electron beam is incident upon the sample surface from an oblique direction with respect to a normal line of the sample and then deflected into a direction of the normal line by an E×B separator to irradiate the sample.
However, such an electron beam apparatus as described above has been had problems, including an increased chromatic aberration in a secondary electron beam caused by the E×B separator, a bad balance of a primary optical system that has been mounted obliquely, and an upper limit of irradiation dose imposed by a fact that the intensity and the emittance of the electron gun are finite and by a further fact that the secondary electron beam is blurred due to space charges from the primary electron beams.
Accordingly, a fourth object of the present invention is directed to solve the above-described problem and a goal thereof is to provide an electron beam apparatus that is free from the limited irradiation dose, the need for the primary optical system and the chromatic aberration of deflection possibly caused by the E×B separator.
The present invention further relates to an electron beam apparatus, and specifically to an electron beam apparatus equipped with a position measuring device for measuring a position of a sample table used in the electron beam apparatus that carries out an evaluation of a sample having a minimum line width not greater than 0.1 μm with high precision and high throughput.
Conventionally, an electron beam apparatus of the above-specified type has been equipped with a position measuring device for measuring a position of a sample table carrying a sample, such as a substrate, for the purpose of irradiating an electron beam to the sample precisely.
Such a position measuring device is operative in such a manner that a laser beam from a single laser oscillator is split into two beams, which are irradiated onto one laser mirror in parallel with x-axis and the other laser mirror in parallel with y-axis, respectively, wherein one laser beam is irradiated from the x-axis side onto the sample table, while the other laser beam is irradiated from the y-axis side onto the sample table so as to measure an irradiation point of an electron beam and the x- and y-directional positions of the sample table accurately.
It has been conventionally recognized that the sample table, during its movement, is subject to the Yaw motion to some extent depending on a tolerance of a stage operable to move the sample table. In the position measuring device of the prior art, in which both of the one laser beam irradiated from the x-axis side to the sample table and the other laser beam irradiated from the y-axis side to the sample table are directed to one and the same optical axis, any misalignment of the sample table slightly offset from an ideal position of the sample table resultant from its Yaw motion could not be a problem but is negligible, so far as only one optical axis is used. However, in the electron beam apparatus having a plurality of optical axes, specifically for a primary electron beam along an optical axis positioned differently from the laser axis along which the laser beam is irradiated, the misalignment would be no more neglected but would problematically induce what is called Abbe's error. Further, disadvantageously, there has been a problem that a movable mirror for the laser is greatly enlarged in size as the sample size becomes larger, and specifically in the case of a stationary mirror for the laser that has been fixedly mounted on a sidewall of a sample chamber, there has been another problem that a measurement error is induced due to an expansion and a contraction of the sidewall of the sample chamber.
Thus the present invention is also directed to solve the above-described problems, and accordingly, a fifth object thereof is to provide a position measuring device for a sample table used in an electron beam apparatus of high precision and high throughput, which eliminates the Abbe's error even in an apparatus having a plurality of optical axes. The present invention further provides an apparatus contemplated advantageously from the viewpoint of preventing growing in size of the sample table due to the movable laser mirror and for eliminating any measurement errors resultant from the expansion and contraction of the sidewall of the sample chamber.
The present invention further relates to a method for evaluating a sample having a pattern defined by a minimum line width not greater than 0.1 μm with high throughput and also to a method for manufacturing a device with high yield by using the same evaluation method.
To carry out a defect inspection of a semiconductor device, a CD (Critical Dimension) measurement and a pattern evaluation including a measurement of alignment accuracy, all by using an electron beam, a method has been conventionally suggested that a multi-beam be formed in a single optical system and a scanning operation be performed with said multi-beam so as to obtain a two-dimensional image.
However, such a technique has not yet been known in the prior art that small sized two-dimensional images obtained from a plurality of detectors corresponding to respective beams are joined together so as to produce a single large-sized image. In this arrangement, the evaluation has been performed as per each beam, and, inefficiently, this needs a complicated procedure.
Further, there has been suggested another technology according to the prior art for producing the multi-beam, in which electron beam emitted from a plurality of emitters are irradiated to apertures equally spaced from an optical axis so as to form a multi-beam.
However, owing to the features of this technology of the prior art characterized by a relatively large spacing between beams of the multi-beam, advantageously it is easier to detect secondary electrons independently, whereas the beams are located relatively distant from the optical axis, and this causes drawbacks including astigmatism and coma aberration in a primary electron beam, problematically inhibiting the primary beam from being converged to be narrow.
Further, in the conventional practice for the CD measurement or the alignment accuracy measurement, the electron beam is converged to be 10 nm or narrower and a thus narrowly converged electron beam is used to measure a line width or a line spacing, thus performing the measurement operation.
However, since a beam current is lower due to a smaller beam size inherent to the prior art, a scanning rate must be reduced in order to obtain a good S/N ratio and thus improve precision, which leads to another problem of the throughput being reduced.
There has been further suggested a method for evaluating a sample by scanning a sample surface with a plurality of beams, including a method using a multi-column and a method using an arrangement of a plurality of beams positioned along a circumference of a circle around an optical axis.
However, the method using the multi-column has a drawback that the throughput would not be improved distinctively since even with a wafer size as large as 12 inches, all that could be contemplated is simply to arrange some columns over the wafer, and at the same time this disadvantageously makes the whole apparatus expensive. Besides, in the method using an arrangement of the multi-beam positioned along the circumference of the circle, it is required to increase the diameter of the circle to generate more beams, and this would inversely intensify the effect of other aberrations than the curvature of field, including the astigmatism and the coma aberration, problematically inhibiting the beam from being converged to be narrow.
Accordingly, a sixth object of the present invention is to provide a method for producing a single large-sized two-dimensional image by joining together a plurality of small-sized two-dimensional images corresponding to a plurality of beams.
Further, a seventh object of the present invention is to provide a pattern evaluation method that is free from any obstacles in converging a primary beam to be narrower, and which can provide an efficient detection of secondary electrons without any cross talks among them.
Further, an eighth object of the present invention is to provide a method for performing a CD measurement and/or measuring an alignment accuracy with high throughput.
Further, a ninth object of the present invention is to provide a pattern evaluation method, in which a multi-beam is generated proximally to a single optical axis, and secondary electrons from respective beams in the concurrent scanning with those beams can be detected efficiently, yet using an electron optical system having a lesser number of lens stage.
The present invention further relates to an evaluation method for evaluating a resist pattern or a subsequently processed wafer, which enables highly accurate and quick evaluation of a lithography margin in a resist pattern written by an electron beam writer and/or in a resist pattern exposed by an ArF, F2 excimer laser stepper.
Conventionally, a defect inspection apparatus employing a light has been used to evaluate the lithography margin or to determine whether or not an optical proximity effect is adequately compensated for. However, in the case where the writer of electron beam direct-writing type or the ArF, F2 excimer laser stepper has been employed as the lithography apparatus, a size of a defect to be detected should be 100 nm or smaller, resulting in a problem of insufficient resolution of the defect inspection apparatus using the light.
In the light of the above problems, a defect inspection apparatus has been suggested that uses an electron beam instead of the light so as to enhance the resolution (see, for example, Patent Document 1 and 2).
[Patent Document 1]
Japanese Patent Laid-open Publication No. Sho 63-17523
[Patent Document 2]
Japanese Patent Laid-open Publication No. Hei 7-249393
Accordingly, a tenth object of the present invention is to provide an evaluation method for evaluating a resist pattern or a subsequently processed wafer, which allows for measurement of a lithography margin with high resolution in a short time with a defect inspection apparatus having a specific performance of minimum size of defect detection not greater than 100 nm.